Dihydroxybenzoquinone as Linker for the Synthesis of Permanently Porous Aluminum Metal–Organic Frameworks

Two new dihydroxybenzoquinone-based metal–organic frameworks, ((CH3)2NH2)3[Al4(L1)3(L1•)3]·3DMF (1, denoted CAU-20) and ((CH3)2NH2)3[Al4(L2)3(L2•)3]·9DMF (2, denoted CAU-20-Cl2), were synthesized at 120 °C in DMF using 2,5-dihydroxy-p-benzoquinone ((C6H2(OH)2(O)2), H2L1) and 2,5-dichloro-3,6-dihydroxy-p-benzoquinone ((C6Cl2(OH)2(O)2), H2L2), respectively. Compared to other Al-MOFs, which contain carboxylate or phosphonate groups that connect the metal sites, in 1 and 2 the Al3+ are coordinated by oxido groups. The metal ions are octahedrally surrounded by oxygen atoms of the deprotonated linker molecules to generate honeycomb layers with a metal to linker ratio of Al: L1/L2 = 2:3. The layers contain L12– and L22– ions as well as linker radical ions L1•3– and L2•3– in a molar ratio of 1 to 1. The presence of radical ions was confirmed by EPR and UV–vis-spectroscopic measurements, and the composition was determined from a combination of PXRD, 1H NMR, TG, and elemental analyses. Charge balance is accomplished through intercalation of (CH3)2NH2+ ions which are formed by partial hydrolysis of DMF. In the structures of 1 and 2 the eclipsed layers are AA and ABAB stacked, respectively, and one-dimensional hexagonal channels with diameters of ca. 9 and 6 Å are formed. Both compounds exhibit permanent porosity and have specific surface areas of 1440 and 1430 m2 g–1, respectively.